Dependence of thermal conductivity of porous silicon on porosity characterized by photoacoustic technique

We have applied a photoacoustic (PA) technique to study the thermal properties of porous silicon (PS) films formed on p-type Si substrates by electrochemical anodic etching with greatly different porosities (20%–60%). From the dependence of the PA signals on the modulation frequency of excitation light measured under a transmission detection configuration, the effective thermal diffusivities for the two-layer PS on Si samples were determined. Using a two-layer model, the thermal conductivity of the PS films was evaluated and found to decrease greatly as the porosity is larger than 30%, which was two or three orders of magnitude lower than that reported for monocrystalline silicon. The results demonstrate that the thermal conductivity of PS films depends strongly on its structure.